1. Field of the Invention
This invention relates to a packaging material for photographic photosensitive materials.
2. Description of Prior Art
Various types of packaging material for photographic photosensitive materials have widely been put to practical use, and various properties are required according to their uses.
As the packaging materials for photographic photosensitive materials, it is necessary to have various properties such as light-shielding, slitability, gas barrier, moistureproofness, antistatic property, rigidity, physical strengths such as breaking strength, tear strength, impact puncture strength, Gelbo test strength and wear resistance, heat sealing properties such as heat seal strength, cut sealability, hot tack properties (hot-seal ability) and seal ability with contrasting material, flatness, elapsed heat seal strength, slipping character and the like. The slitability is the property capable of cutting film smoothly without jagged edges or rupture. This cut sealability is the suitability for forming a fuging seal. In this sealing method, the hot plate for press is a knife-shaped, and melting adhesion and cutting simultaneously occur.
Generally, it is difficult to satisfy these properties by a single material. Therefore, a single layer film of a high-pressure low-density polyethylene (LDPE) kneaded with carbon black or a pigment, and a composite laminated film composed of a LDPE film and a flexible sheet such as paper, aluminum foil or cellophane, etc. have been employed. An example of the conventional film is shown in FIG. 8. This film is a thick single layer film of a light-shielding LDPE resin layer 8a containing carbon black. Another example of the conventional film is shown in FIG. 9. This film was used for packaging a roll of color photographic printing paper, and it is composed of a light-shielding LDPE resin layer 8a, an aluminum foil layer 9, a flexible sheet layer 6, and a light-shielding LDPE resin layer 8a. They are laminated in that order, and an adhesive layer 5 is provided between each layer.
On the other hand, the present inventor has already disclosed a laminated film for photographic photosensitive materials of which physical strength was raised by combing two uniaxially oriented films (U.S. Pat. No. 4,331,725). The inventor has also disclosed a laminated film composed of a foamed sheet having a thickness of 0.3 to 2 mm and an expansion ratio of 5 to 50 times and two uniaxially oriented films laminated on both sides of the foamed sheet so that their molecular orientation axes at larger than 30 degrees (U.S. Pat. No. 4,565,733).
Physical properties of the conventional laminated films are not enough, and during packaging, the films were sometimes torn or hoels made or heat sealing of the films sometimes separated. In addition, when a large amount of a light-shielding material, such as carbon black, was added, physical strengths and heat sealing properties of the film became insufficient. Then, the amount of carbon black was set about 3%, and thickness of the film was more than 70 .mu.m. As a result, the film was stiff, and working efficiency of the packaging process decreased. The cost the packaging was also expensive.
For example, in the case of the laminated film having a 7 layer constitution used as the package for a roll of color photographic printing paper, since the aluminum foil was incorporated in order to secure moistureproofness and antistatic property, physical strengths such as tear strength, impact puncture strength and Gelbo test strength were insufficient. The laminated film disclosed in U.S. Pat. No. 4,565,733 was improved in the physical strengthes such as tear strength, impact puncture strength and Gelbo test strength. However, this laminated film was thick, and antistatic property was insufficient as the packaging material for highly sensitive photographic photosensitive materials. Heat sealing properties were also insufficient in order to secure light-shielding and moistureproofness. Moreover, the ability of being capable of being used in automatic bag making was lacking. Therefore, the heat sealing position was put between heat resistant films, and sealed by using a heat sealer having a particular cooling device.